It's possible noone has experienced trying to remove this style in-tank overflow. If the entire back of the unit is siliconed to the tank glass, it will be difficult to remove, especially if the tank still has water in it.
Did you install it or was it purchased this way?
Any chance of taking a pic from the rear of the unit? It may help generate some answers.
Eheim Pimpette #270
Eheim Pro II 2028; Classics Series -2215, 2217, 2260, 2262; Pro 2229 Wet/Dry